1. Field of the Invention
The subject invention relates to a test apparatus for performing stress and strain analysis on a test object.
2. Description of the Related Art
Engineers and manufacturers endeavor to make products that meet certain performance specifications, while minimizing costs. The size and shape of a product typically affect both the cost and performance of the product.
One way of measuring performance of a product is to assess how the product will perform in response to applied loads. Loads may be applied to a product at various stages through the life of the product, including, during manufacturing, during packaging or shipping and during usage. Data describing the reaction of a product to loads applied at various stages during the product life enable engineers to make more informed design decisions. However, collection of this data often is time consuming and costly. Furthermore test data for small test objects often is not sufficiently accurate or of high enough resolution to be useful.
The prior art includes test equipment that is capable of applying a specified load to a product for a specified duration. The product may be analyzed after the load has been terminated to assess the performance of the product in response to such a load. Tests of this type may be carried out during the prototyping stage of a product development to determine if further design changes may be warranted. Tests of this type also may be carried out on a samples of products from a manufacturing line to assess the ability of manufacturing equipment to produce products in accordance with the specifications. However, prior art machines of this type generally do not account for dimensional changes that may occur in the products in response to the applied load. More particularly, the application of a load to a product will cause some yielding in the product. Thus, the location on the product to which the load had been applied may not be in the exact location that had existed prior to the application of the load. Consequently, the actual load applied to the product may be less than the load specified by a particular test. Other test equipment may be designed to measure dimensional changes in a product in response to applied loads. However, most prior art test devices of this type are not sensitive to relative changes in the applied load that are due to the movement of the parts being measured. Furthermore, the position sensor on most prior art test machines is located on the drive shaft, while the load cell is at the end of an arm that is cantilevered from the drive shaft. Thus, the position sensor does not account for deflection in the cantilevered arm, in the load cell, or in the lead screw assembly.
Most prior art test machines are manufactured for a specific type of test. Thus, portions of the test apparatus that contact a product are substantially dedicated to the specific product being tested. Furthermore, a prior art test apparatus intended for compression analysis typically would not be suited for tensile analysis.
The inventors herein have recognized the need for reliable, rapid and accurate test data in a broad range of industrial and manufacturing environments. For example, test devices could be employed to analyze the forces required to mate two electrical connectors and to consider the yield of electrical contacts in response to various applied loads. In other instances, it may be desirable to assess the force required for puncturing the skin of a patient with a hypodermic needle. The dimensions and bevel angle of the needle may be varied to achieve an optimum puncture. In still other instances, forces and deflection may be analyzed to assess the various laminates of a blister package for tamper proof sealing of medicated capsules. In all of these instances, the loads are small and accuracy is important.
In view of the above, it is an object of the subject invention to provide a test apparatus that can perform a broad range of tests that involve applying loads and measuring applied loads and deflection with great precision.
It is a further object of the subject invention to provide a test apparatus that can perform several types of tests, including tests in compression and tests in tension.
The subject invention is directed to a test apparatus having a base. Any of several stationary work heads may be removably mounted to the base, with the particular stationary work head being selected in accordance with the type of test being carried out and the characteristics of the object on which the test is being performed. For example, a substantially planar stationary anvil may be provided for performing compression tests on a test object having a planar load bearing face. In other situations, the stationary work head may be a non-planar anvil for performing compression tests on an object having a complementary non-planar load bearing surface. In still other situations, the stationary work head may include means for gripping one end of a test object to be analyzed so that an opposed end of the object may be gripped and pulled away from the stationary work head.
The test apparatus of the subject invention further includes a support extending from the base. A drive means may be in or adjacent the support and may extend from the base. For example, the drive means may be a drive screw aligned perpendicular to the top surface of the base. A motor may be mounted in proximity to the base or the support and may be operative to drive the drive means, such as the drive screw.
A movable arm is mounted to the drive means and is selectively movable toward and away from the stationary work head. The movable arm includes an end with means for removably mounting a load cell.
A load cell assembly is mounted to the mounting means of the movable arm. In particular, the load cell assembly comprises a driven end and a sensing end. The driven end of the load cell assembly is removably mounted to the mounting means on the movable arm. The sensing end of the load cell assembly projects from the movable arm. The load cell accurately provides real time information that identifies magnitudes of loads applied by the movable arm. The load cell preferably is calibrated to a sensitivity of about 0.1 gram in compression, in tension or in both.
A movable work head or movable anvil is firmly mounted to the sensing end of the load cell assembly. The particular configuration of the movable work head or movable anvil is selected in accordance with the type of test being performed and in accordance with characteristics of the object on which the test will be performed. Thus, the movable work head may be a substantially planar anvil for performing compression tests on a product having a planar load bearing surface. Anvils of other shapes may be provided for performing compression tests on products that do not have a planar load bearing surface. Alternatively, gripping means may be provided for performing tensile tests.
The apparatus further includes a linear scale mounted to the sensing end of the load cell assembly. The linear scale preferably is parallel to the direction of movement of the movable arm. Thus the linear scale will move with the movable arm and the load cell toward and away from the stationary work head. A read head is fixed in slightly spaced relationship to the linear scale for sensing the magnitude of movement of the linear scale, the movable work head and the sensing end of the load cell relative to the stationary work head. The read head may be a linear encoder that is operative to read indicia on the linear scale precisely. The linear encoder preferably has a sensitivity for measuring dimensional movements of the drive arm of approximately 0.1 micron.
The test apparatus further includes a controller for controlling the operation of the carriage, the load cell and the linear scale. The controller may be operative to ensure that either force or displacement are applied in close agreement to a pre-defined function of each other or a predetermined function of time. For example, the controller may be operative to ensure that a constant load is maintained despite dimensional changes in the object being tested. Furthermore, the controller may be operative to ensure that precise measurements can be made of dimensional changes that are caused by the applied load. The controller may further be connected to a display means, such as a computer screen or a printer. The display means may be operative to tabulate the test results or to graphically present the test results. Thus, for example, the controller may provide real time stress-strain curves to show the way a product reacts to applied loads over time. The display means may be operative to receive input, and hence may be a touch sensitive screen.